1. Field of the Invention
The present invention relates to an underwater sounding apparatus, such as a scanning sonar.
2. Description of the Prior Art
FIG. 12 illustrates a general principles of using a scanning sonar to perform sounding of underwater situations. Referring to the Figure, designated by the letter A is an underwater sounding apparatus installed on a ship S, designated by the letter T is a cylindrical transducer unit of the underwater sounding apparatus A, designated by the letter Bs is a transmitting beam of ultrasonic waves formed by the transducer unit T, designated by the letter Br is a receiving beam for receiving reflected echoes from underwater objects such as fish which reflect the ultrasonic waves, and designated by the letter Z is a water surface. The transmitting beam Bs is transmitted underwater simultaneously in all directions around the transducer unit T, forming an omnidirectional umbrella-like beam pattern directed obliquely downward with a specific tilt angle. On the other hand, the receiving beam Br is a rotating beam having specific directivity. This rotating beam is produced by successively switching elements of the transducer unit T around its outer cylindrical surface, causing the beam to scan over 360xc2x0 at a high speed in a spiral pattern. The receiving beam Br thus produced receives the reflected echoes from the underwater objects, and the information on the underwater situation, such as distribution and movements of fish schools, is obtained by analyzing received signals.
FIG. 10 is a block diagram showing the configuration of functional units of a conventional scanning sonar, which comprises a transducer unit 51, a transmitter 52, a transmit-receive (TR) circuit 53 as well as a receiver section including blocks 54 to 61. The transducer unit 51 transmits ultrasonic waves into the water and receives return echoes. The construction of the transducer unit 51 is conventional, including a plurality of ultrasonic transducer elements arranged on an outer surface of a cylindrical structure. The transmitter 52 produces a pulsed transmitting signal, which has a specific pulselength, and delivers it to the transducer unit 51. The TR circuit 53 is a circuit for switching signal paths between transmit and receive cycles. Specifically, the TR circuit 3 allows the transmitting signal to pass into the transducer unit 1 while prohibiting the transmitting signal from entering the receiver section during each successive transmit cycle and permits only the received signals to enter the receiver section during each successive receive cycle.
Designated by the numeral 54 is a first fixed-band bandpass filter provided in the receiver section, which has a fixed passband. This filter 54 is provided to take out signals in only a desired frequency range from among echo signals received by the transducer unit 51. Designated by the numeral 55 is an amplifier for amplifying the received signals, which have passed through the fixed-band bandpass filter 54. Designated by the numeral 56 is a frequency converter formed of a mixer for converting the amplified received signals from an original ultrasonic frequency to an intermediate frequency. Designated by the numeral 57 is a second fixed-band bandpass filter having a fixed passband for removing unwanted sideband signal components contained in the frequency-converted signals to further narrow the frequency range of the received signals. Designated by the numeral 58 is an amplifier for amplifying the filtered received signals up to a rated input level of an analog-to-digital (A/D) converter 59 provided in a succeeding stage. The A/D converter 59 converts the amplified received signals into a digital signal. Designated by the numeral 60 is a beamformer for forming a rotating receiving beam which successively scans the received signals incoming from all directions around the transducer unit T. Further, designated by the numeral 61 is a detector for detecting an envelope (or a combined echo signal) from a combined received signal produced by beamforming based on a phased array technique. An output of the detector 61 is sent to an indicator which is not illustrated.
In the scanning sonar thus constructed, the transmitter 52 supplies the pulsed transmitting signal of a specific frequency to the transducer unit 51 through the TR circuit 53. The transducer unit 51 converts this pulse signal and emits a beam of ultrasonic waves into the water. The transmitted ultrasonic waves are reflected by fish or other underwater objects and the transducer unit 51 receives return echoes. The transducer unit 51 converts these ultrasonic echo signals into electric signals, which are sent to the receiver section as received signals. The first fixed-band bandpass filter 54 in the receiver section removes undesired noise components contained in the received signals, allowing signal components falling within only the specific frequency range to pass through. The received signals which have passed through the fixed-band bandpass filter 54 are amplified by the amplifier 55 and converted into signals of an intermediate frequency (IF) by the frequency converter 56. Then, the second fixed-band bandpass filter 57 removes sideband signal components from the IF signals. The filtered received signals are further amplified by the amplifier 58 and converted into the digital signal by the A/D converter 59. The beamformer 6D forms a receiving beam which is steered over 360xc2x0 in a spiral pattern. The combined received signal is sent to the detector 61 which detects the envelope of the received signal to reproduce the echo signal. Echo data thus produced is output to the indicator in a succeeding stage. The indicator produces and displays an image of target echoes showing underwater situations on-screen using the data fed from the detector 61.
In the conventional scanning sonar of FIG. 10, the frequency band of the received signal is primarily limited by the first fixed-band bandpass filter 54, and finally defined by the second fixed-band bandpass filter 57. In this respect, it is to be noted that the frequency of the received signal does not necessarily remain constant, but can shift due to factors such as ship speed and motion, tide and waves. FIG. 11 is a diagram illustrating frequency shifts (Doppler shifts) of the received signal caused by a change in ship speed, in which the ship S equipped with the aforementioned scanning sonar is running in the direction of an arrow K. In FIG. 11, (a) to (e) show frequency spectra of different signals. Specifically, (a) shows the frequency spectrum of the transmitting signal transmitted by the scanning sonar, (b) shows the frequency spectrum of a received signal obtained from an echo reflected by a target at point P ahead of the ship S, (c) shows the frequency spectrum of a received signal obtained from an echo reflected by a target at point Q on the right side of the ship S, (d) shows the frequency spectrum of a received signal obtained from an echo reflected by a target at point R behind the ship S, and (e) shows frequency characteristics of the fixed-band bandpass filter 57.
Since the ship S is approaching the point P, the center frequency fa of the received signal from this point P is shifted by f to the positive side from the center frequency fo of the transmitting signal due to the Doppler effect, as shown in (b) of FIG. 11. Also, since the point Q is situated at the same longitudinal location with the ship S (i.e., along its direction of motion), the center frequency of the received signal from this point Q is equal to the center frequency fo, unaffected by the Doppler effect, as shown in (c) of FIG. 11. On the other hand, since the ship S is receding from the point R, the center frequency fb of the received signal from this point R is shifted by f to the negative side from the center frequency fo of the received signal due to the Doppler effect, as shown in (d) of FIG. 11.
As the amount of frequency shift changes with respect to the direction of each individual target relative to the ship S (transducer unit 51), the fixed-band bandpass filter 57 should be of a type having a wide passband w as shown in (e) of FIG. 11 to cover frequency shifts of the received signals from targets in all directions. If the passband of fixed-band bandpass filter 57 is simply widened, however, there arises the problem that the sounding range of sonar decreases due to increases in underwater noise and electrical noise of the sonar system. One potential approach to overcoming this problem would be to increase output power of the scanning sonar. This approach, however, produces another problem in that both the physical size of the sonar and its power consumption increase. While the foregoing discussion has dealt with the frequency shift that occurs in connection with the ship speed, the same discussion applies to frequency shifts occurring due to other external factors, such as ship motion, tide and waves.
In an attempt to overcome the aforementioned problems, Japanese Examined Patent Publication No. 2875118 proposes an ultrasonic sounding apparatus capable of shifting the center frequency of a filter passband according to the amount of Doppler shift, or ship speed, by use of a fixed-band bandpass filter of which center frequency is variable. According to this method, even when the amount of Doppler shift varies, the center frequency and fixed passband of the filter shifts correspondingly and, therefore, it is possible to perform a filtering operation optimized for variations in the frequency of the received signal, thereby maintaining an intended sounding capability.
The sounding apparatus of the aforementioned Patent Publication calculates the amount of Doppler shift from the ship speed and the direction of a sounding beam, and correspondingly shifts the center frequency of the passband of the filter. Therefore, although it is possible to shift the center frequency of the passband to compensate for changes in the ship speed, it is impossible to cancel out frequency shifts caused by other factors, such as ship motion, tide and waves. Because the ship""s hull produces motion on six axes due to waves even when the ship is at anchor, the frequency shifts occur as a consequence. It is, however, impossible to obtain the amount of frequency shift by the prior art method of the aforementioned Patent Publication when the ship speed is zero. Furthermore, when the filtering operation is performed based on the ship speed, it is necessary to continuously obtain ship speed data from external equipment.
In addition, because the passband of the filter is fixed in the aforementioned sounding apparatus, it is impossible to compensate for variations in the frequency range of the received signal when such variations occur from one direction to another, thus imposing limitations on efforts to improve signal-to-noise ratio (SNR). Moreover, because the aforementioned sounding apparatus executes the filtering operation prior to the beamforming operation, it is not possible to make the passband of the filter extremely narrow. Rather, it is necessary to provide multiple stages of fixed-band bandpass filters for reducing the frequency range of the received signal. Providing a number of filters this way results in complexity in circuit configuration and an eventual cost increase.
In light of the aforementioned problems of the prior art, it is an object of the invention to provide an underwater sounding apparatus, which can compensate for frequency shifts of received signals occurring in different individual directions due to various factors, while maintaining a high SNR. Another object of the invention is to provide an underwater sounding apparatus having a capability to controllably shift the bandwidth of echo signals received from all directions using only a single filter.
According to a principal feature of the invention, an underwater sounding apparatus comprises a transducer for transmitting ultrasonic waves into water and receiving ultrasonic echoes reflected from underwater, an indicator for displaying information on underwater situations based on received signals obtained from the ultrasonic echoes, a center frequency detector for detecting the center frequency of the received signal obtained from each direction, a bandwidth detector for detecting the bandwidth of the received signal obtained from each direction, a frequency shifter for shifting the center frequency of the received signal obtained from each direction based on the center frequency detected by the center frequency detector to obtain a baseband signal from the received signal, and a variable passband filter whose passband varies according to the bandwidth detected by the bandwidth detector, whereby the bandwidth of the baseband signal is limited by the passband.
Since the center frequencies of the signals received from the individual directions are determined directly from the received signals, it is possible to cancel out frequency shifts caused not only by changes in ship speed, but also by other factors. Furthermore, because the passband of the variable passband filter varies according to the bandwidth determined from the received signals, it is possible to perform a filtering operation optimized for variations in the frequency range of the received signals even when such variations occur from different directions. This makes it possible to remove undesired noise components contained in the received signals and improve the SNR. Moreover, because the different center frequencies of the signals received from different directions are shifted to the frequency range of the baseband signal, and the bandwidth of the received signals is adjusted by the variable passband filter after fixing the frequency range of the received signals, it is possible to control the bandwidth of the echo signals received from all directions with a single filter without the need for multiple variable passband filters.
According to the invention, the underwater sounding apparatus may further comprise a center frequency estimator for calculating a Doppler shift frequency of the received signal based on ship speed, and estimating the center frequency of the received signal from the calculated Doppler shift frequency. In this embodiment, a first selector chooses whether to use the center frequency detected by the center frequency detector or the center frequency estimated by the center frequency estimator, and the center frequency of the received signal obtained from each direction is shifted to its baseband based on the center frequency chosen by the first selector.
According to the invention, the underwater sounding apparatus may further comprise a bandwidth estimator for estimating the bandwidth of the received signal based on the pulselength of a transmitting signal applied to the transducer. In this construction, a second selector chooses whether to use the bandwidth detected by the bandwidth detector or the bandwidth estimated by the bandwidth estimator, and the passband of the variable passband filter is varied according to the bandwidth chosen by the second selector to adjust the bandwidth of the baseband signal to an optimum range.
The underwater sounding apparatus of the invention may be constructed such that the first selector chooses the center frequency estimated by the center frequency estimator when the accuracy of the center frequency of the received signal detected by the center frequency detector for each direction does not satisfy a specific criterion. This makes it possible to use the estimated center frequency when the center frequency of the received signal cannot be determined with high accuracy due to aeration (air bubbles) in the wake of a ship, for example.
In one form of the invention, the second selector chooses the bandwidth according to a setting entered through an operator terminal. This enables an operator to determine whether to use the bandwidth detected by the bandwidth detector or the bandwidth estimated by the bandwidth estimator.
In one preferred form of the invention, the underwater sounding apparatus further comprises a beamformer which forms a rotating receiving beam for scanning the received signals in an upstream stage of the frequency shifter and the variable passband filter. According to this construction, the bandwidth of the received signal after beamforming operation is narrower than the received signal before the beamforming operation. Therefore, it is possible to make the passband of the variable passband filter considerably narrow and further improve the SNR.
According to the invention, the underwater sounding apparatus may further comprise an audio signal generator which generates an audio signal from an output signal of the variable passband filter. This enables the operator to recognize the presence and movement of fish schools not only visually, but also audibly.
The audio signal generator may include a first audio mixer for inversely shifting the center frequency of the received signal, which has been picked up from a specific audio monitoring direction and frequency-shifted to its baseband (as much as the Doppler shift frequency), and a second audio mixer for shifting the center frequency of an output signal of the first audio mixer to a frequency range best suited as an audio signal. This makes it possible to obtain a xe2x80x9cDoppler-shiftedxe2x80x9d audio output in accordance with the relative moving speed of fish targets.
According to the invention, it is preferable to use a complex autocorrelation method in detecting the center frequency and the bandwidth of the received signal obtained from each direction. This makes it possible to detect the center frequency and the bandwidth of the received signal in real time with a simple circuit configuration.
These and other objects, features and advantages of the invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings.